The present invention relates to a device for calibration of an industrial robot, which calibration device is adapted for being in contact during the calibration with at least one plane of reference arranged on the robot, the device comprising an angle measuring member arranged for measuring an angle relative to the vertical line.
The present invention further relates to an industrial robot comprising at least two links moveably attached to each other by a joint, at least one plane of reference arranged on any of the links, and a calibration device adapted for being in contact with the plane of reference during the calibration.
The present invention also relates to a method for calibration of an industrial robot.
An industrial robot can be viewed as a chain of stiff links. Two adjacent links are joined with each other so that they either are rotatable relative to each other around a rotational axis or linearly displaceable relative to each other. The first link in the chain is the base of the robot and the last link usually constitutes a tool attachment. To be able to determine the position of the robot, each joint usually is provided with an angle measuring device in the form of an encoder or a resolver indicating the position of the joint relative to a zero position. Before an industrial robot can be used it must be calibrated, which means that each of the angle measuring devices is calibrated with reference to the zero position. The robot is calibrated in the production plant before it is delivered and sometimes on site before being set to work. Thereafter, the robot is calibrated after larger reparations such as engine or arm changes.
In the patent document U.S. Pat. No. 5,239,855 a known method of calibration is shown, in which an inclinometer or some other type of instrument for measuring the inclination is used to calibrate the angle measuring devices. An inclinometer measures the angle between an object and the vertical line and can for example be an electronic spirit level. The inclinometer is placed on a plane of reference on one of the links, and generates a signal, which is a measure of the angle between the plane of reference of the link and the vertical line. Thereafter, the joint is moved in dependence of the generated signal until it has a predetermined angle relative to the vertical line. The other links are calibrated in the same way.
The placement of the planes of reference, against which the calibration device is to be attached, is predetermined and is formed by accurately machined surfaces in order to obtain a high degree of flatness. When the robot is to be calibrated, the robot is moved to a predetermined calibration configuration. In this configuration, usually at least one of the links is oriented in a direction which departs from the directions of the other links. Usually, the angles between the length axes of the links are approximately 90xc2x0. Thus, the planes of reference may have different directions depending on which link to be calibrated. The planes of reference are usually either horizontal or vertical during the calibration.
A problem with said method of calibration is that the inclinometer must be mounted on the planes of reference of the robot with a very high precision. Today the inclinometer is first put on an inclinometer plate, which is then mounted on an adapter plate. The adapter plate is then in turn attached to on the plane of reference. Depending on the direction of the plane of reference to be calibrated, different adapter plates are used today. Usually, one type of adapter plate is used for horizontal planes of reference and another type of adapter plate is used for vertical planes. The adapter plate is attached to the robot with screws. This way of attaching the inclinometer to the robot includes a large number of sources of error. Examples of sources of error are mounting errors between the adapter plate and the plane of reference of the robot, mounting errors between the inclinometer plate and the adapter plate, and errors of tolerance of the adapter plate. The fact that several different adapter plates are used also contributes to increasing the mounting error.
A common type of industrial robot comprises a base adapted for resting on a horizontal foundation and a first arm link, which is rotationally arranged relative to the base around a vertical axis. Since the axis being calibrated is essentially parallel with the vertical line, it cannot be calibrated with an inclinometer attached to the first arm link. Accordingly, a second problem with the method of calibration described above is that it cannot calibrate rotational axes parallel to the vertical line.
The object of the invention is to achieve a device for calibration of an industrial robot reducing the number of sources of error during the mounting of the calibration device on the plane of reference.
This object is achieved with the initially defined device characterised in that the device further comprises two contact elements each having a curved surface, and the contact elements are arranged for being in contact with the plane of reference. According to the invention the inclinometer plate is replaced by at least two contact elements. The contact area between a curved surface and a plane surface is considerably smaller than the contact area between two plane surfaces. By decreasing the contact area between the calibration device and the robot, the mounting error decreases as well. Since a measurement of an angle relative to the vertical line is carried out in relation to a straight line, the number of contact elements should be at least two for full accuracy.
In one embodiment of the invention, the curved surfaces are mainly spherical. The spherical surfaces may comprise the whole or a part of a sphere. A spherical surface is advantageous in a manufacturing point of view. For instance, balls from ordinary ball bearings may be used.
In one preferred embodiment of the invention, the number of contact elements is three. With three contact elements one has at least three contact points between the calibration device and the plane of reference of the robot, which means that the angle measuring member and the robot are fixed relative to each other with at least three degrees of freedom. A stable attachment of the calibration device against the robot is thus obtained. The total number of degrees of freedom available for the calibration device and the robot to move in relation to each other is six, (in three directions and around three axes of rotation). The more degrees of freedom the calibration device and the robot are fixed in relative to each other, the more well-defined is the measurement and the smaller becomes the measurement error.
In another embodiment of the invention, the calibration device comprises a first wall element, and the contact elements are arranged for being in contact with the first wall element and the plane of reference. The wall element, which advantageously is firmly attached to the angle measuring member, has a function corresponding to the function of the adapter plate in the prior art.
In another embodiment of the invention, the device comprises a second wall element arranged in an angle relative to the first wall element, wherein said angle essentially corresponds to the angle between two planes of reference of the robot when it stands in a calibration configuration. With such a second wall element, the same calibration device can be used for calibration of planes of reference being arranged in two different directions. Accordingly, there is no need for using different calibration devices or different adapter plates, such as in the prior art, for measuring two planes of reference with different angles relative to the vertical line. The consequence of this is that one source of error, difficult to master during calibration today, directly disappears. The second wall element is preferably arranged essentially perpendicular to the first wall element. If the wall elements are arranged perpendicularly against each other, both horizontal and vertical planes of reference can be measured with the same calibration device.
In another embodiment of the invention, the device comprises four contact elements each having a curved surface, and two of the contact elements are arranged for being in contact with the. first wall element and a first of said planes of reference, and the two others of the contact elements are arranged for being in contact with the second wall element and a second of said planes of reference. By providing both wall elements with contact elements having curved surfaces, the mounting error becomes minimal, independently of which wall element is placed against the plane of reference of the robot.
In one embodiment of the invention, the device contains a second angle measuring member arranged so that its measuring axis differs from the measuring axis of the first angle measuring member. By having two angle measuring members arranged with an angle relative to each other, it is possible to calibrate in two directions without having to rotate the angle measuring member. Advantageously, the second angle measuring member is arranged essentially perpendicular to the first angle measuring member. This implies that two axes being perpendicular to each other can be calibrated without reorienting the calibration device. This is advantageous, for example when the wrist axes are to be calibrated, i.e. the axes 4, 5, and 6, for a robot having six axes. When the robot is in its predetermined calibration configuration, the axes 4 and 6 are parallel to each other and the axes 3 and 5 are perpendicular to the axes 4 and 6. During the calibration of the wrist axes, the calibration device is placed on the outermost link, which is usually a tool attachment. Thanks to the fact that the device comprises two angle measuring members arranged perpendicular to each other, all three axes can be calibrated without moving the calibration device.
In one preferred embodiment of the invention, the calibration device comprises a magnet arranged for attaching the calibration device to the plane of reference of the robot. A magnet keeps the calibration device in place so that well-defined contact areas are obtained between the contact elements and the contact plane of the robot. Accordingly, problems with screws being tightened unequally hard are avoided.
According to one embodiment of the invention, the device comprises a notch arranged for being in contact in at least two points with the curved surface of one of the contact elements. The contact element and the notch together form a mechanical connection. A contact element, which is in contact with a notch in two points, locks two degrees of freedom between the calibration device and the robot. A contact element, which is in contact with a flat surface, only locks one degree of freedom. Thus, by using a notch instead of a smooth surface more degrees of freedom can be locked. From a manufacturing point of view it is an advantage to arrange said notch on the plane of reference of the robot, wherein a corresponding contact element is arranged in connection to the wall element, but it is also possible to arrange the notch on the surface of the wall element and the corresponding contact element on the plane of reference of the robot.
In one embodiment of the invention, the device comprises a notch shaped as a groove having two angled contact surfaces, which are arranged for being in contact in at least two points with the curved surface of one of the contact elements. A groove allows the contact element to move along the groove and accordingly the calibration device may follow any movement in the axis intended to be measured.
In another embodiment of the invention, the calibration device comprises a notch shaped like a prism having three angled contact surfaces, which are arranged for being in contact in at least three points with the curved surface of one of the contact elements. Such a prism locks the angle measuring device relative to the plane of reference in three degrees of freedom.
In a further embodiment of the invention, the calibration device comprises two grooves and two contact elements arranged so that the contact elements are movable in the grooves, wherein one groove and one contact element together constitute a mechanical connection. Two connections lock the calibration device and the robot in four degrees of freedom.
The robot comprises two links moveably attached to each other by a joint. The calibration device is adapted for being positioned across the joint so that a displacement of the links relative to each other becomes measurable. By placing at least one connection on each side of the joint, the calibration device. is enabled to follow the movement of any of the links, and the movement of one link relative to the other link can be measured as a change in inclination of the calibration device. Accordingly, it is possible to measure the change of angle of an axis which is parallel with the vertical axis, and thus the axis can be calibrated. This method may advantageously also be used for calibrating axes not parallel with the vertical axis. Such a movement assumes, of course, that the contact elements are arranged movably along the grooves. Advantageously, the calibration device comprises three grooves arranged in different directions. With three grooves in different directions, all six degrees of freedom become locked, which means that the construction becomes stable. To achieve maximum stability, the grooves are arranged so that their mutual angles are essentially 120xc2x0. Maximum stability means minimum measuring errors.
Another object of the invention is to provide an industrial robot having an calibration device in which the number of sources of error during the calibration is reduced. This object is achieved with the initially defined industrial robot characterised in that the calibration device further comprises two contact elements each having a curved surface, and the contact elements are arranged for being in contact with the plane of reference.
A further object of the invention is to provide a method for calibration of an industrial robot for which the number of sources of error is reduced. This object is achieved by a method comprising the step of bringing a calibration device into contact with a plane of reference on the robot by bringing at least two curved surfaces into contact with at least one surface, measuring an angle corresponding to the angle between the plane of reference and the vertical line and calibrating the robot in dependence of said angle.
The present invention will now be explained by different embodiments described as examples with reference to the appended drawings.
FIG. 1 shows an industrial robot in a calibration configuration.
FIG. 2 shows a first embodiment of a calibration device according to the invention.
FIG. 3 shows a side view of the calibration device in FIG. 2.
FIG. 4 shows a view from below of the calibration device in FIG. 2.
FIG. 5 shows a cross-sectional view of the calibration device along the line Axe2x80x94A in FIG. 3.
FIG. 6 shows a plate having three grooves intended to be connected to three contact elements of the calibration device.
FIG. 7 shows a plate having a groove, a triangular prism, and a surface intended to be in contact with three contact elements of the calibration device.
FIG. 8 shows a plate having two grooves and a surface intended to be in contact with three contact elements.
FIG. 9 shows a cross-section of the calibration device along the line Bxe2x80x94B in FIG. 7.
FIG. 10 shows a second embodiment of a calibration device according to the invention which has two angle measuring devices arranged in different angles.
FIG. 11 shows a third embodiment of a calibration device according to the invention.
FIG. 12 shows a further embodiment of the calibration device according to the invention.